1. Field of the Invention
This invention relates to a subscriber line interface circuit (SLIC) adapted for use in an electronic switching system.
2. Description of the Related Art
A subscriber line interface circuit is used to couple a subscriber terminal, typically a telephone set, with an exchange (an electronic exchange, particularly a digital exchange).
The subscriber line interface circuit is s coupled to a telephone set (subscriber terminal) and an exchange, and handles battery feed or current feed (B), supervisory (S) and hybrid functions (H). The supervisory function supplies a DC current to the telephone set coupled to a telephone line (or a subscriber line) and monitors the status of the handset of the telephone set. That is, the supervisory function monitors whether the handset is in the on-hook state or off-hook state. The battery feed function supplies a DC current to the telephone set to drive an audio exchange for the purpose of telephone communications. The hybrid function is a so-called a 2-to-4/4-to-2 wire conversion function, which terminates the telephone line with a complex impedance that matches with the telephone line set into impedance, extracts a communication signal from the telephone set into the exchange, and transfers a communication signal from the switching device of the exchange to the telephone set via a communications line. The subscriber line interface circuit having these functions may sometimes be called "BSH circuit."
As mentioned above, it is necessary to supply a constant DC current to a telephone set via a telephone line, and at the same time, a voice signal flows through this same telephone line. In order to separate the voice signal, which includes an AC component, from the constant DC current and prevent the AC component from subscriber line interface circuit that comprises a hybrid coil and a switching current is provided with a large inductance coupled in series to a DC power source. With this design, the DC current is supplied to the telephone set via the inductance element. In order to prevent the DC current from leaking onto the telephone line, thus preventing the DC component from being superimposed onto the voice signal, a capacitor of as large as 2 .mu.F is coupled in series to each of a pair of telephone lines so that the voice signal is transferred via this capacitor to the hybrid circuit for 2-to-4/4-to-2 wire conversion.
However, the subscriber line interface circuit of this type needs relatively large inductance element and capacitor. In this respect, this subscriber line interface circuit is not suitable to make the electronic exchange compact, reduce the manufacturing cost of the subscriber line interface circuit or realize its large integration.
In circuit calculation or design, the line impedance of a telephone line is typically treated to be 600.OMEGA.. A standard telephone set is designed such that it would have the best side tone characteristic when coupled to a subscriber line interface circuit through a telephone line whose length is set to provide a line
loss of 5 dB. However, in the case of a private branch (PBX), the telephone line has a relatively short length and its allowed line loss is reduced to 2 dB or below. Since a subscriber line interface circuit is used in such a private branch exchange, its terminal impedance and/or balance network impedance is demanded to have a complicated characteristic that should also include the cable characteristic of the telephone line.
As already mentioned, the conventional subscriber line interface circuit needs a large inductance and/or capacitor to attain a predetermined impedance, so that this restriction significantly hinders the realization of a compact exchange. To overcome this problem, it has been proposed to realize the functions of the traditional subscriber line interface circuit using an electronic circuit. However, this electronic circuit needs a number of circuit elements and is not therefore effective in reducing the size and the manufacturing cost of the subscriber line interface circuit.
An example of such an electronic subscriber line interface circuit is disclosed in Japanese Patent Disclosure (Kokai) No. 58-104558. For example, FIG. 3 of this publication shows a series circuit of a terminal resistor (31) and a capacitor (32) of a subscriber line, which gives a predetermined value to the impedance of the subscriber line interface circuit as measured from the side of the subscriber terminal. The system with such a series circuit requires that the capacitor should have a large volume or a large size. This stands in the way to reduce the size of the system (the subscriber line interface circuit). In addition, the conventional circuit necessitates that a resistor denoted by 52 in FIG. 3 or 68 in FIG. 4 of the aforementioned Japanese publication should have a highly accurate absolute value and should be constituted by a discrete element for securing the necessary accuracy. Since the above capacitor and resistors are constituted by discrete elements, not all the constituent elements of the conventional subscriber line interface circuit can be integrated in a circuit if tried. Therefore, enlarging the overall exchange system cannot be avoided.
The number of subscriber line interface circuits in use increases as the number of lines that the exchange handles increases. This means that reduction in size and manufacturing cost of the overall exchanger system cannot be realized without reducing the size and the manufacturing cost of the subscriber line interface circuit itself.
As mentioned above, a conventional electronic circuit requires elements large in size and number and also a complex control in order for the circuit to provide the terminal impedance and/or various functions, such as the current feed, supervisory and hybrid functions, necessary for a subscriber line interface circuit.